The present invention is generally directed to control strategies for internal combustion engines, and in particular to the control of the engine idle speed. Although the present invention will in the main be described in relation to engines having direct injected fuel injection systems, it is to be appreciated that the invention is also applicable on engines using alternative fuelling systems.
The Applicant has developed dual fluid direct injected fuel injection systems for use on both two and four stroke internal combustion engines. An example of such a dual fluid fuel injection system is described in the Applicant""s U.S. Pat. No. 4,693,224, the contents of which are incorporated herein by reference. These fuel systems can be used in a wide variety of recreational, marine, automotive and aeronautical engine applications.
Such directed injected engines are typically controlled by varying the fuelling rate to the engine as a function of engine load and speed. When the engine is operating at idle, an idle controller including a Proportional Integral Differential (PID) system is typically used to return or maintain the operation of the engine at a predetermined base idle speed. The idle controller generally provides closed loop engine idle speed control by varying the fuelling rate to the engine so that the engine speed is maintained and returned to the base idle speed. When initially coming out of an off-idle mode of operation and into idle, the idle controller may achieve this by setting engine speed set-points which progressively ramp the engine speed down to the final base idle speed.
Operation of the idle controller is normally initiated when the engine speed drops below a predetermined level as it approaches idle. This predetermined level is known as the xe2x80x9cidle entry set-pointxe2x80x9d. Once the engine speed falls below this idle entry set-point speed, the idle controller then acts to reduce the engine speed by varying the fuelling rate to the engine to progressively bring the engine speed to the final base idle speed. The idle controller then maintains the engine speed at this steady state base idle speed while the engine is in its idle or no load state. Hence, the idle controller is typically initialised at a certain offset above the base idle speed and this is primarily done to ensure that a smooth transition from off-idle engine speeds to idle is possible.
It has been found for certain engine applications that it may not always be practical to use an idle speed control strategy wherein only a single idle entry set-point has been defined. This may particularly be the case in respect of vehicles having low inertia engines and/or a continuously variable transmission (CVT) such as, for example, scooters and all-terrain-vehicles (ATVs). In such vehicles, the CVT may not de-clutch from the engine at the same speed at which the clutch engages. Therefore, situations can arise whereby the engine can enter the idle control process with either the CVT engaged or disengaged. Hence, in one possible scenario, while the engine may be in an unloaded or xe2x80x9czero demandxe2x80x9d state, the engine may still continue to be driven through the CVT. The result of this is that the rate of engine speed deceleration is lower than would normally be the case if the engine had been de-clutched. As a result, the engine speed may, for an extended period of time, be maintained at a significantly higher level than the set-point or final base idle speed.
If the engine is not de-clutched from the vehicle drive-train, the idle controller cannot satisfactorily control the engine speed down to the set-point base idle speed. The idle controller, in trying to control the engine speed down to the base idle speed, will typically react to the large error between the actual idle speed and the base idle speed by significantly reducing the engine fuelling rate. However, once the engine is finally de-clutched from the drive-train of the vehicle, with little to no fuel present in the fuel system due to the previous efforts of the idle controller, it is difficult to prevent the engine speed from dropping well below the base idle speed. This can result in significantly reduced torque backup and typically stalling of the engine.
This is generally the case if a relatively high idle entry set-point is defined in the idle speed control strategy. Whilst this problem may be partially addressed by instead adopting a relatively low idle entry set-point (ie: such that the error between the actual engine speed and the set-point or base idle speed is comparatively small when there exists no demand on the engine and it is being driven through the CVT), this then introduces other problems. In particular, where the engine is de-clutched from the vehicle drive-train and the rate of engine speed deceleration is quite high, the adoption of a low idle entry set-point will typically not give the idle controller enough of an opportunity to ensure that undershoot of the set-point engine speed and/or stalling does not occur.
Accordingly, certain applications exist where it may be beneficial to have two or more idle entry set-points such that satisfactory idle speed control can be effected by the idle controller in response to a number of different scenarios.
It is therefore an object of the present invention to provide an improved method of controlling the engine idle speed which ameliorates at least some of the above noted problems.
With this in mind, according to one aspect of the present invention, there is provided a method of controlling the idle speed of an internal combustion engine including, in response to the engine speed being below a predetermined level: determining the rate of change of the speed of the engine; selecting an idle entry set-point as a function of said rate; and initiating idle speed control of the engine on the basis of said idle entry set-point to thereby control the engine speed to a base idle speed.
Preferably, the rate of change of the speed of the engine is determined after it has been established that the engine speed is decelerating. Preferably, the rate of change of speed is determined following a reduction in the engine speed to a point below the predetermined level. Conveniently, the rate of change of engine speed is determined once it has been established that an off-idle to idle transition is occurring in the operation of the engine. In this way, it is ensured that the rate of change of engine speed is only determined when it is apparent that the engine will soon be seeking to enter an idle mode of operation.
Preferably, idle speed control of the engine speed is performed in a closed loop manner. In this regard, any suitable idle speed controller may be adapted for use with the method of the present invention.
Conveniently, during closed loop idle speed control of the engine, engine speed set-point curves may be set and the engine speed controlled to follow the set-point curves so as to progressively reduce the engine speed down to the base idle speed. The idle speed controller is typically embodied in an electronic control unit (ECU) which manages the operation of the engine. Such ECU""s are well known in the field of engine management and as such will not be elaborated on further herein. Conveniently, the idle speed controller will comprise a xe2x80x9cPIDxe2x80x9d system which serves to determine the error between the actual engine speed and the set-point speed to facilitate close loop idle speed control for the engine. The idle speed controller may however incorporate or use any other appropriate system including, for example, a xe2x80x9cPIxe2x80x9d or xe2x80x9cPxe2x80x9d system.
Conveniently, the rate of change of the engine speed may be determined as an idle entry gradient, the gradient increasing with increasing rate of change of engine speed. Hence, where a rapid deceleration in engine speed were to occur due to, for example, a short sharp burst of the vehicle throttle whilst at standstill, the gradient would typically be quite sharp (ie: large). In contrast, where a lower rate of deceleration of the engine speed were to occur, for example, where the engine is still clutched to the vehicle drive-train but there is no driver induced demand or load thereon, the gradient would be noticeably flatter (ie: small).
Preferably, at least one high idle entry set-point and one low idle entry set-point speed may be predetermined for use with the idle controller. Both the low and high idle entry set-point values will generally be greater than the base idle speed.
Conveniently, the high idle entry set-point is selected to be at a level which would avoid the engine speed undershooting the base idle speed following a rapid deceleration of the engine speed. That is, the high idle entry set-point is preferably selected such that a smooth transition from off-idle to idle may occur subsequent to a large rapid reduction in the engine speed.
Conveniently, the low idle entry set-point is selected to be at a level which would enable the engine speed to be controlled down to the base idle speed within a reasonable period following a gradual deceleration of the engine speed. More particularly, the low idle entry set-point is selected to be at or near the point at which the engine would normally be expected to be de-clutched from the vehicle drive-train. For example, if the CVT of a scooter was known to de-clutch from the engine at say 2000 rpm following a deceleration, the idle entry set-point may conveniently be selected to be at any point below 2000 rpm.
Preferably, when the idle entry gradient is above a predetermined gradient value (i.e. steeper or sharper) idle speed control is initiated on the basis of the high idle entry set-point speed. In this way, the idle controller is able to prevent undershooting of the base idle speed by the engine speed and ensure a smooth transition into idle. The relative greater difference between the base idle speed and the high idle entry set-point allows the idle controller enough opportunity to gain control of the engine speed which may be falling quite rapidly.
Preferably, when the idle entry gradient is below the predetermined gradient value (i.e. flatter) idle speed control is initiated on the basis of the low idle entry set-point speed. In this way, the large error which would otherwise occur between the actual engine speed and the base idle speed if the engine has not de-clutched from the vehicle drive-train would be avoided. Hence, when the engine finally does de-clutch, the idle controller is able to gain control of the engine speed and avoid the engine stalling or misfiring.
Conveniently, the gradients of the engine speed set-point curves vary as a function of the selected idle entry set-point value. In this way, the actual engine speed at the selected idle entry point can more closely correspond to the gradient of the speed set-point curves set by the idle controller to enable a smoother transition into idle.
Conveniently, a greater number of different idle entry set-points may be preset to provide for more precise idle speed control in response to different rates of deceleration of the engine speed.
Alternatively, the idle entry set-point may be variable over a range of set-point values. That is, the idle entry point may be calculated by a suitable algorithm or function based on the determined idle entry gradient. Hence, in this way, the most optimum idle entry point for a range of possible speeds would be able to be selected in response to the particular idle entry gradient at the time.
Conveniently, the rate of change of engine speed or the idle entry gradient are only calculated when the engine speed is between a predetermined band or range of engine speeds above the base idle speed. Preferably, the lower end of the speed range is preset to correspond with a point above the high idle entry set-point. Preferably, the speed range is set at an appropriate point above the high idle entry set-point. Conveniently, the upper end of the speed range corresponds to the predetermined value below which the rate of change of engine speed is calculated. In this way, rather than waiting for the engine speed to reduce to idle or near idle, the idle entry gradient is able to be determined when it is anticipated that the engine speed is returning to idle.
Preferably, idle speed control of the engine is not initialised until there is zero demand on the engine. That is, even though the idle entry gradient may be calculated in anticipation of the engine speed dropping to idle, the gradient is not used to determine which of the idle entry set-points is used to initiate idle speed control unless there is no load demand on the engine. Zero demand may relate to any situation where the vehicle operator has closed or released the throttle such as would be the case whilst idling prior to a launch or coasting down a hill without fuelling the engine (i.e. over-run cut conditions).
Preferably, the engine is one in which fuel is delivered directly to the combustion chamber(s) of the engine. The idle speed control method discussed above may have particular applicability to such direct injected engines due to the typical faster transient response properties thereof. That is, as fuel is delivered directly into the cylinder(s) of the engine, changes in engine speed due to increases or decreases in the engine fuelling rate are effected in comparatively shorter times.
Conveniently, fuel is delivered to the engine by way of a dual fluid fuel injection system wherein a metered quantity of fuel is propelled into the combustion chamber(s) by way of a source of compressed gas.
Such a system is disclosed for example in the Applicant""s U.S. Pat. No. RE 36768, the contents of which are included herein by way of reference.
Still further, the idle speed control method is preferably effected in an engine which is controlled according to a fuel-led control system over at least part of its operating range, that is, rather than the operator demand dictating the level if airflow to the engine from which the fuelling rate is subsequently determined, the operator""s demand in a fuel-led control system directly dictates the fuelling level for the engine. Accordingly, any changes to the engine operating speed and the engine idle speed are effected by way of varying the engine fuelling rate in contrast to varying the airflow to the engine. Such a fuel-led control system is discussed, for example, in the Applicant""s U.S. Pat. No. 5,540,205, the contents of which are included herein by way of reference.
According to another aspect of the present invention, there is provided an ECU adapted to control the idle speed of an internal combustion engine, including in response to the engine speed being below a predetermined level:
determining the rate of change of the speed of the engine;
selecting an idle entry set-point as a function of said rate; and initiating idle speed control of the engine on the basis of said idle entry set-point to thereby control the engine speed to a base idle speed.
It will be convenient to further describe the invention with respect to the accompanying drawings which illustrate a preferred embodiment of the method of the present invention. Other preferred embodiments of the invention are also envisaged, and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.